1. Field of the Invention
The present invention relates to an isolation or gate valve for a single crystal pulling apparatus opening and shutting a passage between a main chamber and a pull chamber of the single crystal pulling apparatus.
2. Description of the Related Art
A single crystal pulling apparatus in accordance with the Czochralski method of pulling a single crystal rod from a melt of a raw crystal , e.g., polycrystal comprises a cup-shaped crucible holding a raw crystal and placed in the main chamber, a hollow cylindrical heater surrounding the crucible, and a hollow cylindrical heat insulator surrounding the heater. The single crystal pulling apparatus further comprises a hollow cylinder defining the pull chamber joined with the top end of the hollow cylinder defining the main chamber. A single crystal rod which is pulled from the melt into the pull chamber is taken out of the pull chamber. An isolation valve is provided in a passage between the main chamber and the pull chamber for opening and shutting this passage.
Japanese unexamined patent application publication HEI.2-252687 (Patent Application No.1-70692) discloses an example of a prior art isolation valve. FIG. 4 shows this example. The prior art isolation valve 1 comprises a valve body or shutter 2 in the form of a circular tray. The valve body 2 shuts and opens a cylindrical outlet 3a extending upwards from a flange 3 integrally disposed on the top of a hollow cylinder defining the main chamber. The exterior cylindrical surface of the outlet 3a has an O-ring 4 mounted thereon. The interior surface of the rim of the valve body 2 is in or out of sealing contact with the O-ring 4.
The valve body 2 is connected to a valve rod 6 by means of an arm 2a fastened by screw bolts to the valve body 2. The upper end of the valve rod 6 is connected to a piston 7a of an air cylinder 7 to move the valve rod 6 up and down. A joint between the piston 7a of the air cylinder 7 and the valve rod 6 has an arm 5 extending outside a casing of the isolation valve 1 for rotating the valve rod 6 and thereby the valve body 2. Thus, the valve rod 6 and the valve body 2 are movable up and down and rotatable in the casing of the isolation valve 1.
When the interior surface of the rim of the valve body 2 is in sealing contact with the O-ring 4 to shut the outlet 3a, the valve body 2 isolates the main chamber from the pull chamber. On the other hand, when the valve body 2 is rotated from the position shown in FIG. 4 to open a space above the outlet 3a, the isolation valve 1 enables the pull chamber to communicate with the main chamber.
The isolation valve 1 is employed in order to isolate the main chamber from the pull chamber when a single crystal (e.g., silicon single crystal) rod that has been pulled from a polycrystal melt held in the crucible placed in the main chamber which is taken out of the pull chamber. Employing the isolation valve enables continuously charging raw polycrystal in a melt while taking a pulled single crystal rod outside a pull chamber essentially without any intrusion of the outside air into the main chamber
Alternatively, employing the isolation valve enables the sequent steps of placing a raw polycrystal charger in the pull chamber after a step of taking the single crystal rod out of the pull chamber, exhausting a gas out of the pull chamber to vacuum or substituting the gas in the pull chamber with an inert gas, and opening the isolation valve so that the raw polycrystal charger can additionally charge the raw polycrystal in the melt of the crucible.
If the isolation valve 1 isolates the main chamber from the pull chamber when the single crystal rod is taken out of the pull chamber after the single crystal rod has been pulled from the melt, the crucible holding the polycrystal melt in the main chamber can be at a high temperature while the main chamber is under vacuum or the atmosphere of an inert gas (e.g. gaseous argon) and the interior of the single crystal pulling apparatus can be protected from pollution by the outside air.
In a process of the sequent steps of opening the pull chamber to the outside air while the main chamber is under a reduced pressure, taking the single crystal rod out of the pull chamber and replacing a gas in the pull chamber with vacuum or the inert gas, the pressure of the main chamber happens to exceed the pressure of the pull chamber. In this case, a gas leaks from the main chamber to the pull chamber unless the isolation valve 1 securely seals the outlet 3a. This abnormally reduces the pressure of the main chamber, or by chance the replacing inert gas may diffuse from the pull chamber to the main chamber thanks to an excess in pressure of the replacing gas due to poor control. The diffusion of the replacing inert gas pollutes the atmosphere of the main chamber to disturb the pulling of the single crystal rod and adversely affect the quality of the single crystal rod.
When the isolation valve 1 shuts the outlet 3a, the operating arm 5 rotates the shaft 6 and the valve body 2 is turned from the space distant from the outlet 3a back to the space above the outlet 3a, and the air cylinder 7 depresses the valve body 2 to cap and seal the outlet 3a. In order to isolate the main chamber from the pull chamber, the air cylinder 7 is required to hold the capping of the valve body 2 on the outlet 3a.
The apparatus of continuously pulling the single crystal rod closes the door in the sidewall of the cylinder defining the pull chamber and then exhausts the gas out of the pull chamber to make the pull chamber vacuum, or repeats the steps of making the pull chamber vacuum and subsequent introducing the inert gas into the pull chamber after the single crystal rod is taken out of the pull chamber.
In the isolation valve 1, the assembly of the arm 2a and the valve body 2 cantilevers from the shaft 6, so that it is difficult that the air cylinder 7 drives a free or remote end of the valve body 2 to securely cap and seal the outlet 3a when the isolation valve 1 is required to fully isolate the main chamber from the pull chamber. Since the pressure of the pull chamber becomes negative to the pressure of the main chamber when the pull chamber is made vacuum, the pressure difference between the pull chamber and the main chamber may turn up the free end of the valve body 2. Once the free end of the valve body 2 is turned up, the sidewall of the outlet 3a and the free end of the valve body 2 produces a gap to impair the isolation between the pull chamber and the main chamber. The gas leak from the main chamber to the pull chamber causes the above problems. In particular, if the diameter of a semiconductor wafer sliced from the single crystal rod will be increased from now on and the diameter of the outlet 3a will accordingly increased, it is important to solve the above problems.